even if they were highly resistant, they ought to screen the gas. They
certainly screen it perfectly in a condition of rest, but not by far
perfectly when the charge is surging in the coating. But the loss of
energy which occurs within the tube, notwithstanding the screen, is
occasioned principally by the presence of the gas. Were we to take a
large hollow metallic sphere and fill it with a perfect incompressible
fluid dielectric, there would be no loss inside of the sphere, and
consequently the inside might be considered as perfectly screened,
though the potential be very rapidly alternating. Even were the sphere
filled with oil, the loss would be incomparably smaller than when the
fluid is replaced by a gas, for in the latter case the force produces
displacements; that means impact and collisions in the inside.

No matter what the pressure of the gas may be, it becomes an important
factor in the heating of a conductor when the electric density is
great and the frequency very high. That in the heating of conductors
by lightning discharges air is an element of great importance, is
almost as certain as an experimental fact. I may illustrate the action
of the air by the following experiment: I take a short tube which is
exhausted to a moderate degree and has a platinum wire running through
the middle from one end to the other. I pass a steady or low frequency
current through the wire, and it is heated uniformly in all parts. The
heating here is due to conduction, or frictional losses, and the gas
around the wire has--as far as we can see--no function to perform. But
now let me pass sudden discharges, or a high frequency current,
through the wire. Again the wire is heated, this time principally on
the ends and least in the middle portion; and if the frequency of the
impulses, or the rate of change, is high enough, the wire might as
well be cut in the middle as not, for practically all the heating is
due to the rarefied gas. Here the gas might only act as a conductor of